Olga Smirnova

High harmonic interferometry of multi-electron dynamics in molecules

High harmonic emission occurs when an electron, liberated from a molecule by an incident intense laser field, gains energy from the field and recombines with the parent molecular ion. The emission provides a snapshot of the structure and dynamics of the recombining system, encoded in the amplitudes, phases and polarization of the harmonic light. Here we show with CO2 molecules that high harmonic interferometry can retrieve this structural and dynamic information: by measuring the phases and amplitudes of the harmonic emission, we reveal ‘fingerprints’ of multiple molecular orbitals participating in the process and decode the underlying attosecond multi-electron dynamics, including the dynamics of electron rearrangement upon ionization. These findings establish high harmonic interferometry as an effective approach to resolving multi-electron dynamics with sub-Ångström spatial resolution arising from the deu2009Broglie wavelength of the recombining electron, and attosecond temporal resolution arising from the timescale of the recombination event.

Anatomy of strong field ionization

Strong field ionization is a starting point for a rich set of physical phenomena associated with attosecond u2009science. This paper provides an introductory overview of the basic theory of strong field ionization and focuses on (i) the physics and the dynamics of the electron transition to the continuum, and (ii) the shape of the electron wavepacket as it appears in the continuum.

Reading diffraction images in strong field ionization of diatomic molecules

We present a theoretical analysis of how intense, few-cycle infrared laser pulses can be used to image the structure of small molecules with nearly 1 fs temporal and sub-A spatial resolution. We identify and analyse several physical mechanisms responsible for the distortions of the diffraction image and describe a recipe for recovering an un-distorted image from angle and energy-resolved electron spectra. We also identify holographic patterns in the photoelectron spectra and discuss the requirements to enhancing the hologram resolution for imaging the scattering potential.

Coulomb–laser coupling in laser-assisted photoionization and molecular tomography

Using the example of laser-assisted photoionization, we analyse the interplay of an intense laser field and the atomic/molecular potential during the electron motion after ionization. We give conditions to determine when the electrons oscillations in the strong laser field are approximately decoupled from its acceleration in the ionic potential, and when they are not. Excellent agreement between analytical and numerical results allows us to assess the recipes for analysing interference structures in high harmonic generation in molecules.

Time- and frequency-resolved coherent anti-Stokes Raman scattering spectroscopy with sub-25 fs laser pulses

In general, many different diagrams can contribute to the signal measured in broadband four-wave mixing experiments. Care must therefore be taken when designing an experiment to be sensitive to only the desired diagram by taking advantage of phase matching, pulse timing, sequence, and the wavelengths employed. We use sub-25 fs pulses to create and monitor vibrational wavepackets in gaseous iodine, bromine, and iodine bromide through time- and frequency-resolved femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy. We experimentally illustrate this using iodine, where the broad bandwidths of our pulses, and Boltzmann population in the lower three vibrational levels conspire to make a single diagram dominant in one spectral region of the signal spectrum. In another spectral region, however, the signal is the sum of two almost equally contributing diagrams, making it difficult to directly extract information about the molecular dynamics. We derive simple analytical expressions for the time- and frequency-resolved CARS signal to study the interplay of different diagrams. Expressions are given for all five diagrams which can contribute to the CARS signal in our case.

Tunability of THz Emission Originating from Sub-Cycle Electron Bursts in a Laser Induced Plasma

Since the demonstration a decade earlier by Cook and Hochstrasser [1] of intense THz emission from air ionized with a two-color (ω + 2ω) laser field, this phenomenon remains in the focus of attention both as a source of intense single-cycle THz pulses and because the underlying mechanism(s) of THz emission are hotly disputed. Experimental evidence points toward both the four-wave mixing mechanism based on stationary or nonstationary third-order susceptibilities [2] and toward the model of micro electric currents in which electrons experience directional drift in an “AC bias” of the broken-symmetry electric field [3]. To date, only commensurate ω + 2ω schemes were in use, which corresponds to a degenerate case and thus complicated the delineation of different emission mechanisms. In this contribution we show that by fixing the carrier envelope phases and detuning the frequencies of two optical fields the timing of sub-cycle optical field ionization in gas can be controlled yielding continuous frequency tuning of the emitted THz radiation. Our results suggest that this particular mechanism of THz emission is closely related to two prominent phenomena, ATI and HHG generation that are also based on quasi-periodic sub-cycle ionization followed by electron acceleration in the E-field.

Tunable THz generation with a CEP-stable muticolor OPA

THz emission tunability is demonstrated in a plasma driven by a field synthesized with a multicolor CEP stable OPA. Sub-cycle field ionization followed by continuum-continuum electron transitions are responsible for tunable low frequency emission.

Bound States Of A Free Electron: The Role Of The Kramers-Henneberger Atom In The Higher-Order Kerr Effect

We discuss the connection between strong-field ionization, saturation of the Kerr response, and the formation of the Kramers-Henneberger (KH) atom and long-living excitations in intense external fields.

Observing the Birth of attosecond pulses

Attosecond pulse trains are produced during the production of high harmonics when a gas sample is irradiated with an intense femtosecond laser pulse. Usually the attosecond pulses are characterized in a second gas source. We show that attosecond pulses can be measured directly as they are produced by using a weak second harmonic of the driving laser, and that such a field can also be used to control the production process. We demonstrate manipulation of the harmonic spectrum as well as the creation of transient diffractive elements in the nonlinear medium itself.

Strong-Field Ionization of Molecules: Simple Analytical Expressions

We use the method developed in [1] to derive simple expressions for strong-field ionization rates of molecules, as a function of their alignment with respect to the electric field. Our analytical expressions offer transparent physical interpretation, and demonstrate the connection between the geometry of the ionizing molecular orbital and alignment-dependent ionization rates, including the sensitivity of the angular patterns to the field strength. We also show how static tunnel ionization rates can be generalized to describe the subcycle ionization dynamics in oscillating laser fields.